Metals are known to rust on exposure to moist environment. Organic coatings have long been used for corrosion protection due to their barrier properties. Coatings that provide active corrosion inhibition such as zinc rich coatings and chromates, phosphates and the like, have been the mainstay of the industry for many years. The zinc rich coatings provide cathodic protection, while the chromates and phosphates are believed to passivate the metal. Innumerable inorganic pigments and fillers have been so claimed to provide corrosion protection. Multiple coatings are often necessary to overcome non-uniformities and pin holes that are the source of corrosion in organic coatings. Growing concerns over the adverse toxicological properties of corrosion inhibiting pigments, has prompted the industry to look for alternatives. Intrinsically conductive polymers (ICPs) such as polyanalines offer such an alternative. The interest in ICPs as corrosion inhibitors is based on the following properties--ICPs passivate the metal by forming a protective oxide layer; ability of the conducting polymer coating to tolerate pin holes and scratches; and the redox chemistry, that provides for continuous repairing of the oxide layer in case of scratches and dents.
Intrinsically conductive polymers are a relatively new class of material. The use of ICPs for corrosion protection was proposed over a decade ago by A. G. MacDiarmid, Personal Communication at the Int. Conf. Synth. Metals, 1986, Kyoto, Japan. Since then several groups around the world have demonstrated the effectiveness of ICPs in corrosion inhibition of metals. The ICP technology is fundamentally different in its approach to corrosion prevention compared to conventional organic coatings, in that ICPs take part in the electrochemical corrosion reaction.
However, it was not until the early to mid-nineties that detailed investigation on corrosion protection by polyaniline coating was conducted. All of the previous disclosures and references of the potential use of conjugated polymers refer to the use of continuous of conductive coating by electrochemical polymerization directly on the metal surface. Thompson et al (K. G. Thompson, D. J. Bryan, B. C. Benicewicz, D. A. Wrobleski, Los Alamos National Laboratory Report LA-UR-92-360) reported that mild steel coupons coated with continuous coating of neat solutions of polyaniline in 1-methyl-2-pyrrolidone provided significant corrosion inhibition in 3.5% sodium chloride and 0.1M hydrochloric acid solutions. Wessling (Adv. Mater., 6(1994)226) was the first to report passivation of metals using dispersions of polyaniline. Polyaniline was deposited from pure polyaniline dispersions on metallic samples. The dip coating procedure was repeated 5-20 times to provide thicker coatings. Wessling observed a significant positive shift in corrosion potential along with reduction in corrosion current. Upon removal of the polyaniline coating, Wessling also observed change in appearance and the presence of passivated layer was confirmed.
Lu et al., (Lu. W. K. Elsenbaumer. R. L., and B. Wessling, SYNATH. MET. 71(1995) 2163, and Wei-Kang Lu, Sanjoy Basak and Ronald L. Elsenbaumer. "Corrosion Inhibition of Metals by Conductive Polymers" HANDBOOK OF CONDUCTING POLYMERS, edited by Terje A. Skotheim, Ronald L. Elsenbaumer and John R. Renyolds, Marcell Dekker (1998), reported corrosion protection of mild steel in acidic and saline atmosphere using neutral and doped polyaniline coatings, with a epoxy top coat. Neutral polyaniline was applied from NMP solutions, which were further doped with p-toluene sulfonic acid. Both the neutral and doped polyanilines showed corrosion protection. Corrosion protection provided by doped polyaniline was more significant in acid conditions than saline conditions. Very recently, Sitaram et al (S. P. Sitaram, J. O. Stoffer and T. J. O'Keefe, Journal of Coatings Technology, 69(866), 1997,65) have reported corrosion protection of untreated steel using Versicon, a doped polyaniline, neutral polyaniline and PANDA, a soluble form of polyaniline manufactured by Monsanto. They reported PANDA exhibited significant improvement in corrosion protection, when used as a base coat, with a conventional top coat. It was interesting to note that both Versicon and PANDA did not exhibit significant protection, when formulated in to conventional coatings such as epoxy or acrylics. They concluded that polyaniline/PANDA does not function as a pigment.
In summary, conductive polymers, especially polyanilines have been shown to provide corrosion protection to steel. However, there is discrepancy as to the extent of corrosion protection and the effect of the form of the polyaniline and the nature of corrosion environment. Electrochemical techniques are not suitable for many industrial applications. The level of corrosion protection of chemically prepared polyaniline seems to be extremely dependent on the formulation and is unreliable. While Wessling reports passivation of metals using polyaniline dispersions and blends, Sitaram et al have disclosed that blends of polyaniline with conventional resins, were less effective than neat coatings. Further, commercial application of ICPs is hampered by commercial non-availability and the cost of these materials.
Commercial application of ICP materials is hampered by their processability, commercial availability and the cost of these materials. Their effectiveness in corrosion inhibition is dependent on the chemistry and the method of preparation. Results vary considerably with the method of preparation.
Therefore there remains a need for corrosion inhibitive coatings that can overcome the adverse toxicological properties of corrosion inhibiting pigments, such as the chromates, and overcome the processing and cost disadvantages of inherently conductive polymers and which can provide consistent corrosion protection.
Corrosion protective coatings based on intrinsically conductive polymers (ICP's) can have a wide range of commercial applications such as bridges, rebars used in concrete, underground storage tanks, ships, oceanic drilling platform equipment, automotive and several industrial machinery, equipment and metal furniture. In order for polyaniline and other ICP's to be successful commercially in corrosion prevention, it is apparent that they need to be applied as coatings using practical techniques. Further, these coatings need to be environmentally attractive. In addition to processability of ICP's, the coatings must provide excellent adhesion to the substrate metal, be durable and environmentally stable in certain applications. Thus, there is a clear need for a ICP coating formulated to offer the properties described above that can provide enhanced corrosion protection.